Coating of protected electrocatalytic material on an electrode

ABSTRACT

A coating of a protected electrocatalytic material on a dimensionally stable anode or other electrode.

This application is a continuation of application Ser. No. 144,907,filed May 19, 1971, now abandoned, which in turn is acontinuation-in-part of application Ser. No. 702,695, filed Feb. 2,1968, now U.S. Pat. No. 3,632,498.

When dimensionally stable electrodes formed from valve metals, such astitanium, tantalum, zirconium, niobium, etc. and alloys thereof, areused as anodes for chlorine production in electrolysis cells and inother electrolysis processes, it is necessary to provide a coating orpartial coating for such dimensionally stable anodes which is capable ofconducting current from the metal base of the anode to the electrolyteand of catalyzing the formation of chlorine molecules from the chlorideions released at the anode. Such coating also prevents passivation ofthe anode over long periods of time.

Typical conductive electrocatalytic coatings are gold, silver, platinum,palladium, iridium, ruthenium, osmium, rhodium, iron, nickel, chromium,copper, lead, manganese, and oxides, nitrides, carbides and sulfidesthereof and graphite. However, when said coatings are applied to adimensionally stable anode base and subjected to the conditions of achlorine electrolysis cell, whether of the mercury cathode or diaphragmtype, the said coatings deteriorate or are consumed or destroyed rapidlyunder the cell conditions.

One of the objects of this invention is to provide a method ofprotecting said coatings on a dimensionally stable anode base and ofprolonging the effective life of said electrocatalytic material on theanode base.

Another object of the invention is to provide a method for protectingthe electrocatalytic material on an electrode which utilizes relativelyinexpensive materials in the coatings thereon and which is neverthelessexcellent for carrying out electrolytic processes and provides anelectrode having a long life and is stable in operation.

Various other objects and advantages of this invention will appear asthe description proceeds.

The invention will now be described in greater detail with reference tothe accompanying drawings, in which:

FIG. 1 is a cross-section of the electrode according to the invention;and

FIG. 2 is a graph in which the performance of prior electrodes iscompared with that of an electrode according to the invention.

Referring to the drawings, the electrode according to the inventionconsists of a base or core 10 having a coating 11 thereon, which twoparts consist of material which will be described more fullyhereinafter.

The electrode is shown as having a simple rectangular shape, but it willbe understood that the electrode is not limited to such a configuration,but may have any configuration suitable for the electrolysis apparatusin which the electrode is to be used. Furthermore, there is shown asimple cavity 12 at the top for connecting the current conductorthereto, but this feature does not constitute part of the invention andmay be changed as desired.

The base or core of the electrode according to the invention consists ofa conductive material which, at least on the outside, is resistant tothe electrolyte in which it is to be used. Thus, for example the basemay consist of any of the film-forming metals such as aluminum,tantalum, titanium, zirconium, bismuth, tungsten, niobium, or alloys oftwo or more of these metals. However, I may use other conductivematerials which will not be affected by the electrolyte and the productsformed during the dissociation thereof, it being possible to use metalssuch as iron, nickel or lead, and non-metallic conductive materials,such as graphite, in suitable electrolytes.

The coating 11 consists of an electrically conductive electrocatalyticmaterial such as one or more metals, oxides, nitrides, carbides orsulfides of the electrolytic coatings recited above, together with aprotecting coating material for protecting said conductiveelectrocatalytic material from the conditions encountered in a chlorineor other electrolysis cell. While there are a number of theories as tothe exact nature of the coating, it is thought that the protectingcoating material may surround the electrocatalytic material and protectit from the conditions in the cell while not inhibiting theelectrocatalytic activity of the electrocatalytic material. Theprotecting material may consist of oxides, nitrides, carbides andsulfides of titanium, tantalum, zirconium, niobium, aluminum, lead or ofgraphite. It should preferably be coprecipitated from a mixed solutionor suspension of the protective material and a solution from which anelectrocatalytic material from the group consisting of gold, silver,platinum, palladium, iridium, ruthenium, osmium, rhodium, iron, nickel,chromium, lead and manganese can be precipitated. The mixed solution orsuspension may be formed of soluble salts of the metals forming thefinal coating or of mixed suspensions, said solutions precipitating thesalts together in intimate mixture in the coating on the electrodes. Thematerials may be applied in separate coats.

The coating according to the invention need not cover the entire surfaceof the electrode to be immersed in the electrolyte. As a matter of fact,the coating need only cover 2% of the immersed zone, and the electrodewill still operate effectively and efficiently.

There are a number of methods of forming the protected electrocatalyticcoating on the base to produce the mixed coating material. The mostpractical one thereof comprises the coprecipitation of an oxide of afilm-forming metal with the other material of the mixture constitutingthe coating, which coprecipitation may be effected chemically,thermally, electrically, or by a combination of these methods. Onemethod of effecting such a coprecipitation consists in preparing asolution containing materials from which one or more oxides of theprotective film-forming metal can be precipitated, and further materialsfrom which the electrocatalytic conductor material can be precipitatedand thereafter treating the solution in such a manner that the oxide oroxides of the film-forming metal are coprecipitated with the conductorsof the electrocatalytic material. Among the methods of treating thesolution are evaporation of the solvent followed by the thermaloxidation of the mixed material, whereby when the solution is firstapplied to the surface of the electrode to be coated by a treatment suchas brushing, immersion, or spraying, the coprecipitated mixture remainsbehind on the surface of the electrode. Alternatively, the pH of thesolution can be so adjusted that the materials of the mixture areprecipitated to form a suspension and then the portion of the electrodeto be coated can be immersed in the suspension and an electrophoresiseffected to precipitate the materials onto the electrode. Such a methodis preferably followed by sintering to promote the adhesion of thedeposited mixture to the material of the core of the electrode.

A particular method of coprecipitating the materials to form the mixedcoating material comprises preparing a solution containing a solvent anda soluble compound or compounds of a film-forming metal, which willprecipitate when the solvent is evaporated, and a soluble compound orcompounds of a non-film-forming electrocatalytic conductor which willalso precipitate when the solvent is evaporated. The solution is appliedto the surface of the electrode base to be coated, and the base thuscoated is heated one or more times, preferably several times, in anon-reducing atmosphere.

Alternatively, only one of the materials in the solvent need beevaporated. That is to say, either a compound from which an oxide of afilm-forming metal can be deposited, or a compound from which anon-film-forming conductor can be deposited, the other compound orcompounds being suspended in the solution. The subsequent treatments arethe same as in the case that all materials are in the dissolved state.

A different method of making the electrode consists in the use of theso-called vacuum-sputtering techniques, in which the base is placed in avacuum and cathods of one or more film-forming metals are placed in thevacuum together with a cathode of an electrolytic non-film-forming metalor an oxide thereof, and the sputtering current is conducted through ananode and the cathodes so that the electrolytic film-forming metalprotective oxide or oxides are sputtered onto the base together with theelectrolytic non-film-forming metal or electrocatalytic metals or oxideor oxides thereof.

Still another method of making the electrode according to the inventionconsists in the use of an electrolysis. The base of the electrode isimmersed in an electrolyte consisting of a solution of salts or othercompounds of one or more protective film-forming metals from whichsolution the oxide or the oxides will coprecipitate onto the electrodewhen the solution is subjected to electrolysis. The solution alsocontains a soluble compounds of a non-film-forming electrocatalyticmetal or metals or of an oxide or oxides of such metals which will alsocoprecipitate during the electrolysis. The electrolysis can be effectedeither by passing an alternating current through the electrode, or byusing the electrode as an anode and conducting a direct current throughit.

Generally speaking, the formation of the mixtures of the oxidesaccording to the invention can be effected thermally by heating in theair, but in some cases this can be beneficially affected by conductingthe heat treatment under subatmospheric or superatmospheric pressure.The heating may be effected by resistance heating or high-frequencyheating.

When the mixtures are applied electrolytically, this is best effectedunder anodic conditions, and preferably so that one or more hydroxidesof the metals are deposited on the base, such hydroxides beingsubsequently sealed by boiling in demineralized water or by heating.

Generally speaking, the starting products are salts of the metals whichare converted into the desired oxides thermally. The acid residue ispreferably so selected that the salt is converted into an oxide at atemperature of from 400°-1200°C. I preferably use acid residues ofvolatile acids such as HCl, HBr, or acetic acid.

The manner in which the electrode according to the invention is usedwill be readily apparent to those skilled in the art. For most uses, theelectrode is placed as an anode in an electrolysis apparatus, and theelectrolysis is carried out in the conventional manner and underconventional conditions, the product or products of the electrolysisbeing yielded in the conventional manner, or the purified electrolytebeing recovered, as desired. Examples of processes in which theelectrode is thus used are the electrolysis of brine in mercury cells ordiaphragm cells for the production of chlorine and alkali metalhydroxides, the electrolytic production of chlorates, hydrochlorites,persulphates, and perborates, the electrolytic oxidation of organiccompounds, such as liquid or gaseous hydrocarbons, for example,propylene or ethylene, the electrolytic deposition of metals,desalination of water, sterilization of water, and fuel cells. Theelectrode is also excellently suitable for use as an anode in cathodeprotection systems and as a cathode in bi-polar cells.

While the electrocatalytic ingredient of the electrode coating issometimes referred to herein as a chlorine discharge catalyst, capableof catlyzing the formation of chloride ions into chlorine molecules andof discharging the chlorine molecules from the surface of an anode intothe electrolyte, it will be understood that electrodes coated with anelectrocatalytic material and a protective compound therefor are used inmany other electrolysis process, as described above.

As explained above, the provision of the mixed coating containing theconductive electrocatalytic material and the protecting material is theparticular feature accounting for the outstanding performance of theelectrode according to my invention. The importance of the restrictionthat the coating must behave as a mixed material in which theelectrocatalytic ingredient is protected by the protecting coatingmaterial, rather than as mere physical mixture of the two materials inwhich they act separately, can be shown by means of several examples.Iron oxide itself is highly sensitive to hydrochloric acid at roomtemperature, and so are several titanium oxides. I have found, however,that when a co-precipitated mixture of iron oxide and titanium oxide isapplied to a base of conductive material, it is only affected byhydrochloric acid at room temperature to a very small extent. Similarly,ruthenium oxide coated on a titanium base, connected as an anode in analkali metal chloride electrolysis, which anode is contacted with theamalgam formed in a mercury cell, loses a part of its thickness after aprolonged period of electrolysis, because the reductive properties ofthe amalgam convert the ruthenium oxide into metallic ruthenium, and themetallic ruthenium is readily dissolved in the amalgam from the surfaceof the titanium and is not resistant to the electrolyte. Co-precipitatedmixed oxides of titanium oxide and ruthenium oxide, however, which arein contact with such an amalgam, are resistant to the amalgam becausethese mixed oxides are not reduced and so do not dissolved in theamalgam or in the generated chlorine, as the titanium oxide protects theconductive electrocatalytic ruthenium oxide.

It should be noted that the mixed materials which are applied to theelectrodes according to the present invention are quite different fromthose obtained, for example, by mere heating in the air of the solidnoble metals, or when these are superimposed in discontinuous layers infinely divided condition on other metals. Generally speaking, it may besaid that the oxidation of the solid metals by mere heating is verydifficult, and that although finely divided noble metals may beoxidized, the adhesion of such oxides to the substrate is often verypoor. Electrolytic oxidation is also very difficult, and in addition,layers produced in this manner also show poor adhesion, so that amechanically weak electrode is formed. The problem of rendering theoxides of the noble metals and other metals in finely divided conditionadhesive and at the same time protective and resistant is now solved byvirture of the coprecipitation of the non-film-forming electrocatalyticconductors with the oxides of the film-forming metals. It is suprising,for example, that palladium oxide, platinum oxide and ruthenium oxideare fully resistant to chlorine cell conditions, when protected bycoprecipitated titanium oxide. This is not so with platinum plateapplied to a metallic base, either by electro-deposition orchemi-deposition, when heated in the air or used as an anode in theelectrolysis of alkali metal chloride. The platinum plate is notresistant to chlorine cell conditions and does not acquire the conditionrequired according to the present invention. That is to say, an adheringmixture is coprecipitated thereon.

The following Table A clearly indicates the difference between theco-precipitated protected oxides according to the present invention andthe other oxides which may be formed thermally or electrolytically when,for example, an electrode consisting of a base of metallic titanium anda coating of a platinum metal is oxidized in the air or used toelectrolyze a dilute solution of an alkali metal chloride or dilutehydrochloric acid.

    ______________________________________                                        CHEMICAL AND ELECTROLYTIC PROPERTIES OF SINGULAR OXIDES AS                    COMPARED WITH THE MIXED PROTECTIVE OXIDES ACCORDING TO THE                    INVENTION.                                                                                   THERMAL OXIDIATION IN AIR AT                                                  500°C OF Pt/Pd/Ag/Fe/Ru IN                                             FINELY-DIVIDED CONDITION ON                                                   TITANIUM BASE.                                                 ______________________________________                                        Formation of oxide         N/B/B/B/G                                          Adhesion to base metal     B/B/B/B/B                                          Resistance to 0.2% sodium amalgam                                                                        B/B/B/B/B                                          Overvoltage in chlorine electrolysis                                           at 8000 amps/m.sup.2      B/B/--/--/B                                        Loss of oxides per tone of chlorine                                            at 8000 amps/m.sup.2      M/M/--/--/M                                        Chemical resistance to aqua regia                                              without current           B/B/--/--/B                                        Resistance to reduction    B/B/B/B/B                                          Catalytic properties in the oxidation                                          of organic compounds      B/B/B/B/B                                          Mechanical strength        B/B/B/B/B                                          ______________________________________                                         KEY: E -- Excellent                                                           B -- Bad                                                                      G -- Good                                                                     N -- Practically no Oxide Formed                                              M -- Much                                                                     L -- Very Little.                                                        

    ______________________________________                                                      Electrolytic oxidation in dilute                                              sulphuric acid of Pt/Pd/Ag/Fe/Ru                                              in finely-divided state on                                                    titanium base                                                   ______________________________________                                        Formation of oxide     N/B/B/--/G                                             Adhesion to base metal --/--/B/--/B                                           Resistance to 0.2% sodium amalgam                                                                    --/B/B/--/B                                            Overvoltage in chlorine electrolysis                                                                 --/B/--/--/G                                           at 8000 amps/m.sup.2   (for a short time)                                     Loss of oxides per ton of chlorine at                                         8000 amps/m.sup.2      --/M/--/--/M                                           Chemical resistance to aqua regia                                             without current        --/B/--/--/B                                           Resistance to reduction                                                                              --/B/--/--/B                                           Catalytic properties in the oxidation                                         of organic compounds   --/B/B/B/B                                             Mechanical strength    --/B/B/B/B                                             ______________________________________                                    

             Coprecipitated Oxides of Ru/Ti, PT/Zr,                                        Pd/Ta, Ag/Ti, Fe/Ti, and Pt/Ti In Finely-                                     Divided State On Titanium Base, In Which                                      The Valve Metal Oxide Acts As A Protective                                    Coating For The Electrocatalytic Platinum                                     Group Metal Oxide.                                                   ______________________________________                                        Formation of oxide       E/E/E/E/E/E                                          Adhesion to base metal   E/E/E/E/E/E                                          Resistance to 0.2% sodium amalgam                                                                      E/E/E/--/E/E                                         Overvoltage in chlorine electrolysis                                          at 8000 amps/m.sup.2     E/E/E/--/--/E                                        Loss of oxides per ton of chlorine at                                         8000 amps/m.sup.2        L/L/L/--/--/L                                        Chemical resistance to aqua regia                                             without current          E/E/E/--/--/E                                        Resistance to reduction  E/E/E/--/--/E                                        Catalytic properties in the oxidation                                         of organic compounds     E/E/E/--/--/E                                        Mechanical strength      E/E/E/--/--/E                                        ______________________________________                                    

In a test designed to show quantitatively the improvement obtained byelectrodes according to the invention as compared with other electrodesin contact with 0.2% sodium amalgam under a constant electric load of10.000 Amp/m² during electrolysis and of 80.000 Amp/m² duringshort-circuiting with the amalgam, a number of titanium bases wererespectively coated with (1) metallic ruthenium, (2) a mixture ofplatinum and iridium (70/30 weight/weight), (3) a coprecipitated mixtureof ruthenium oxide and titanium oxide (90 mol%/ 10 mol%), and (4) acoprecipitated mixture of ruthenium oxide and titanium oxide (30 mol%/70mol%). All of these materials were present in a thickness of 10 g/m².The electrodes were introduced into a brine test cell containing 0.2%sodium amalgam, which quantity was kept constant, the brine in the cellhaving a concentration of 28% and a temperature of 80°C, the appliedcurrent density being 10.000 Amp/m². These conditions are the same asmay be the case in a large-scale cell. FIG. 2 shows the overvoltage inmillivolts plotted against the time. It will be seen that, whereas theovervoltage of the first, second and third electrodes increasedrelatively rapidly owing to the contact with the amalgam, and noprotection for the electrocatalytic platinum group material, theovervoltage of the electrode according to the invention, i.e.,containing more than 50% titanium oxide, only increased gradually duringa long period of time.

The following tables show the protective effect of titanium oxide of theelectrocatalytic material when a mixture of titanium oxide and rutheniumoxide is applied to a titanium electrode base and tested (a) by dippingunder load into a 0.2% sodium mercury amalgam (which dipping frequentlyoccurs in mercury cell operation), and (b) in operation in a mercurychlorine cell.

    ______________________________________                                        (a)                                                                           WEIGHT LOSS AFTER 5 min. DIP IN 0.2% AMALGAM UNDER LOAD                       25.000 amp/m.sup.2. WEIGHT OF RuO.sub.2 IS 10 gr/m.sup.2 OF ANODE             ______________________________________                                        SURFACE.                                                                      RuO.sub.2 /TiO.sub.2                                                                        90/10 mol %     10.7 gr/m.sup.2                                 RuO.sub.2 /TiO.sub.2                                                                        70/30 mol %     8 gr/m.sup.2                                    RuO.sub.2 /TiO.sub.2                                                                        50/50 mol %     6 gr/m.sup.2                                    RuO.sub.2 /TiO.sub.2                                                                        30/70 mol %     0.75 gr/m.sup.2                                 RuO.sub.2 /TiO.sub.2                                                                        10/90 mol %     1 gr/m.sup.2.                                   ______________________________________                                    

    ______________________________________                                        (b)                                                                           WEIGHT LOSS AFTER 60 DAYS RUNNING IN A MERCURY CELL.                          WEIGHT OF RuO.sub.2 IS 10 gr/m.sup.2 OF ANODE SURFACE                         ______________________________________                                        RuO.sub.2 /TiO.sub.2                                                                        90/10 mol %     7 gr/m.sup.2                                    RuO.sub.2 /TiO.sub.2                                                                        80/20 mol %     6 gr/m.sup.2                                    RuO.sub.2 /TiO.sub.2                                                                        70/30 mol %     5 gr/m.sup.2                                    RuO.sub.2 /TiO.sub.2                                                                        60/40 mol %     2.75 gr/m.sup.2                                 RuO.sub.2 /TiO.sub.2                                                                        50/50 mol %     2.50 gr/m.sup.2                                 RuO.sub.2 /TiO.sub.2                                                                        45/55 mol %     1.5 gr/m.sup.2                                  RuO.sub.2 /TiO.sub.2                                                                        30/70 mol %     0.4 gr/m.sup.2                                  RuO.sub.2 /TiO.sub.2                                                                        10/90 mol %     0.4 gr/m.sup.2.                                 ______________________________________                                    

The invention is illustrated, but not limited, by the followingexamples.

EXAMPLE I

6.2 cc butyl alcohol

0.4 cc HCl 36%

3 cc butyl titanate

1 g RuCl₃

The solution was several times brushed on to a cleaned titanium plate(grain size titanium 0.04-0.06 mm; ASTM 6) of 10 × 10 cm and a thicknessof 1 mm, the plate being first pickled in hot aqueous oxalic acid,subjected to ultrasonorous vibration in water, and dried. The plate thustreated was heated in the air at a temperature of 300° - 500°C for 1 - 5minutes.

The resulting electrode had a coating of ruthenium oxide coprecipitatedwith titanium oxide, the titanium oxide being present in a proportion of70 mol %, the balance being RuO₂.

The resulting electrode was placed in a hydrochloric acid cell as ananode, the cathode being a silver plated titanium electrode. Flowinghydrochloric acid of 25% was electrolyzed at 70°C and 2500 Amp/m² forpractically one year with excellent results and with losses of less than0.1 g ruthenium per ton of chlorine.

The resulting electrode was placed in a brine electrolysis cell as ananode, the cathode being mercury and the brine having a concentration of28% a pH of about 2.5, and a temperature of 80°C. The spacing betweenanode and cathode was less than 2.5 mm. When a current density of 10.000Amp/m² was applied, the anode had an extremely low overvoltage of about80 millivolts, measured against a calomel reference electrode, and thiswas maintained for a long period of time, even after variousshort-circuitings with the amalgam.

The resulting electrode was placed in a brine diaphragm cell as ananode, the cathode being iron, and the brine having a concentration of28%, a pH of about 3.5, and a temperature of 80°C. At a current densityof 1000 Amp/m², the anode had an extremely low overvoltage of 60 mVoltsand maintained this for a long period of time. The losses of metallicruthenium were less than 0.15 g per ton of produced chlorine, in themercury cell and less than 0.1 g of produced chlorine in the diaphragmcell.

The resulting electrode was also used in a cathodic protection system asanode for the protection of a ship. The electric design was aconventional system well-known to those skilled in the art. The anodeshowed good electrical and mechanical properties.

The resulting electrode was extremely suitable for the oxidation ofunsaturated organic compounds, such as ehtylene and propylene, as wellas for the preparation of chlorates.

The resulting electrode was also suitable for electrodialysis, becauseit readily admits of pole changing.

The resulting electrode was also used in a galvanic metal depositionprocess, in which gold was deposited on coper from a bath having thefollowing composition: gold choride 30 g/l, nitric acid (specificgravity 1.19) 25 cc/l, sodium chloride 12 g/l, sulphuric acid (specificgravity 1.025) 13 g/l (+ organic brighteners). By means of this bath, at70°C, and a current density of 8-10 Amp/m², an excellent plating on thecathode was obtained, the overvoltage at the anode being such that nodamage was done to the bath.

EXAMPLE II

80 cc TiCl₃ -solution in H₂ O (25% TiO₂)

1 g RuCl₃

This mixture was absorbed in a graphite anode at a subatmosphericpressure this anode being previously subjected to ultrasonorousvibrations for 10 minutes. Subsequently the anode was heated in a streamof air for 1/2 hour at a temperature of 300° - 800°C. This treatment wasrepeated four times. The resulting electrode had a coating of rutheniumoxide, co-precipitated with titanium oxide, the titanium oxide beingpresent in a proportion of 98.4 mol % TiO₂, there being 1.6 mol % RuO₂.

An untreated graphite anode was placed in an alkali metal chloride cellcontaining 28% brine of a pH of about 2.5 and a temperature of 80°C asthe electrolyte and a mercury cathode. The distance between the anodeand the cathode was less than 2.5 mm.

A current of a density of 8.000 Amp/m² was passed through the cell. Theanode first had an overvoltage of about 400 mV, which decreased to 360and after a considerable time increased to 450 mV. Furthermore, theuntreated anode showed marked erosion after a short while, and as aresult the brine solution became black with the graphite released. Inaddition to the contamination of the bath liquid, the loose graphitecaused stray currents, resulting in loss of efficiency and discharge ofthe amalgam. Furthermore, the spacing between the anode and the cathoderequired adjustment at regular intervals, because this spacing changedas a result of the erosion of the anode, resulting in loss of energy inthe electrolyte.

The electrode according to this example, placed in the same electrolyteunder the same conditions, had an overvoltage of only 70 mV, whichovervoltage remained constant during a considerable time. Moreover, thebath remained clear and the anode showed no erosion. Accordingly, notonly was the electrolyte not contaminated, but the electrodes did notrequire adjustment.

The electrode according to the invention was also used as an anode in acathode protection system of a conventional type and operatedexcellently.

EXAMPLE III

A tantalum plate was cleansed well and mechanically roughened, and acoating mixture was prepared as follows:

18 cc isopropyl alcohol

1 g iridium chloride

2 g platinum chloride

4 g isopropyl titanate

3 cc anise-oil (reducing agent)

Lavender oil or linalool may be used instead of the anise-oil.

The mixture was brushed onto the tantalum plate several times, and thecoated base was subsequently heated at a temperature of 600°C, forseveral minutes. The resulting electrode had an oxide coating of iridiumand platinum, co-precipitated with titanium oxide, the titanium oxidebeing present in a proportion of 65.8 mol % in addition to 12.65 mol %iridium and 21.55 mol % platinum.

This electrode operated excellently in electrolytic processes for thepreparation of chlorine, oxygen, oxidation of organic compounds, and ingalvanic baths.

EXAMPLE IV

A zirconium plate was degreased and a coating mixture was prepared asfollows:

10 cc water

1 g gold chloride

3 cc 25% titanium chloride solution

0.1 cc wetting agent

This mixture was brushed onto the degreased plate and the plate washeated in the air at a temperature of 200° - 300°C and at asuperatmospheric pressure. This treatment was repeated 8 times.

The resulting electrode had a coating of gold oxide coprecipitated withtitanium oxide, the titanium oxide being present in a proportion of 74mol % and the gold oxide in a proportion of 26 mol %.

This electrode operated excellently in dilute sulphuric acid solutions.

EXAMPLE V

A titanium rod was degreased and then pickled for 8 hours in a 10%oxalic acid solution at 90°C. The rod was subsequently brushed with thefollowing mixture:

30 cc TiCl₃ solution in water

3 g anhydrous ferric chloride

1 g ferrous chloride

The resulting rod was subsequently heated in a space filled with amixture of steam and air at a temperature of 450° -600°C for 1-2 hours.

The resulting rod was connected in a cathodic protection system. Theelectrode operated excellently in alkaline solutions at currentdensities upto 1,000 Amp/m².

EXAMPLE VI

6.2 cc butyl alcohol

0.4 cc hydrochloric acid 36%

1 g zirconium acetyl acetonate

1 g iridium chloride, dry

The solution was applied to a zirconium base as described in Example I,the base being previously degreased, pickled, and subjected toultrasonorous vibrations. After the application of the solution the basewas heated at 500° -700°C for several minutes by clamping the basebetween two copper plates heated throughout their surface. This resultedin a highly uniform heating of the overall surface, which was highlybeneficial to the quality of the anode. The treatment was repeatedseveral times. The ratio of zirconium oxide to iridium oxide in themixture had been so selected that more than 50 mol % of zirconium oxidewas present in it. The anode thus made was excellently suitable for allkinds of electrolytic processes, particularly for the electrolysis ofsulfuric acid solutions and solutions of sulfates.

EXAMPLE VII

9 cc butyl alcohol

0.4 cc hydrochloric acid 36%

1 g palladium chloride

3 cc pentaethyl tantalate.

A tantalum base was dipped into the above solution and after dryingheated at 500° - 800°C to deposit a mixture thereon of 62 mol % tantalumoxide and 38 mol % palladium oxide. This treatment was repeated sixtimes. The tantalum base was a thin tube which after the completion ofthe coating was provided with a copper rod acting as a currentconductor, because the tantalum tube comprised insufficient metal for itto be able to transport current without undue losses. In order to ensureproper contact between the tantalum tube and the copper rod, the innersurface of the tantalum tube was electrolytically copper-plated.Intimate contact between the copper rod and the copper inner coating wasobtained by applying molten tin therebetween and allowing the tin tosolidify.

The anode made in this manner was excellently suitable for cathodicprotection purposes with an applied voltage of higher than 20 volts, andalso is an excellent anode for the preparation of hypochlorites.

EXAMPLE VIII

6.2 cc butyl alcohol

0.4 cc hydrochloric acid 36%

1 g ruthenium chloride `3 cc niobium pentaethylate.

A niobium base was degreased and connected as an anode in an electrolyteto form an oxide coating thereon. This coating was subsequently rinsedthoroughly and dried. The anode with the oxide coating thereon wasdipped into the above solution and subjected to high-frequency heatingat 600°C at a subatmospheric pressure of 100 mm Hg to convert thereactants to the desired mixture. This treatment was repeated severaltimes until the desired mixture was present on the niobium in athickness of 2 microns.

The anode thus made was excellently suitable for all kinds ofelectrolytic processes, such as for the preparation of chlorine,chlorates, and hypochlorites, for the sterilization of swimming-pools,etc..

EXAMPLE IX

A titanium plate was degreased and pickled and subsequently an oxidecoating of about 1 mm thickness was applied to it by means ofelectrolysis.

A mixture of: `10 cc butyl alcohol

1 g ruthenium oxide powder

3 cc butyl titanate

was painted onto it and converted into the desired mixture at atemperature of 300° - 600°C. This treatment was repeated so many timesthat 10g/m² of the desired mixture was present on the surface of thetitanium plate.

The anode made in this manneer was excellently suitable for theelectrolytic preparation of chlorine, and chlorine compounds, and forcathodic protection purposes.

The electrolytically formed oxide on the titanium highly promotes theadhesion of the mixture formed.

EXAMPLE X

A niobium expanded metal plate was pre-treated in known manner andsubsequently brushed with a solution of:

10 cc water

1 g ruthenium chloride

1/2 cc hydrochloric acid (35%)

2 g titanium hydroxide.

The plate was subsequently heated at 400° - 700°C for several minutesuntil the desired mixture formed. This treatment was repeated until 6g/m² of the mixture was present on the surface.

This anode was excellently suitable for the electrolysis of alkalinesolutions.

EXAMPLE XI

An aluminum plate was degreased and pickled in a conventional manner.There was then prepared a mixture of:

10 cc isopropyl alcohol

1 g aluminum bromide

1 g platinum chloride

0.01 g iodine.

The aluminum plate was dipped into this mixture and heated at 400°C toform the required mixture, the latter consisting of 62.2 mol % A1₂ 0₃and 37.8 mol % Pt0₂.

This treatment was repeated several times, the mixture being applied tothe plate either by dipping or painting.

The electrode thus made is excellently suitable for the electrolysis ofboric acid compounds.

EXAMPLE XII

The following mixture was prepared:

10 cc butyl alcohol

6 cc butyl titanate

2 g graphite (can be replaced by titanium nitride or tantalum carbide orrhenium sulfide)

This mixture was painted onto a titanium base and heated at atemperature of 400°-700°C, which treatment was repeated a number oftimes.

An anode thus coated with graphite and titanium oxide is particularlysuitable for electrolyses in which a low current density is desirable,for example, cathodic protection of subterraneous objects.

Anodes in which the coating contains in addition to titanium oxide anitride, carbide, or sulfide are resistant to high current densities invarious electrolytes.

EXAMPLE XIII

2 g titanium chelate

1 g ruthenium chelate

These two chelates were intimately admixed in the dry state andsubsequently placed on the bottom of a vessel which can be closed andheated. A degreased, pickled titanium rod, covered as to 98% with aheat-resistant silicon lacquer layer, was introduced into the vessel. Byheating the chelates, a mixture of titanium oxide and ruthenium oxidewas evaporated onto the 2% of exposed titanium, and the required crystalform was obtained by sintering. A small quantity of hydrochloric acidvapour in the vessel promotes the adhesion of the mixed oxide.Subsequently the lacquer layer was removed. The resulting electrode hasan active surface area of about 2%.

This electrode is excellently suitable as an anode for the sterilizationof water in swimming-pools or for the electrolysis of two layers ofliquid, in which a local electrolysis of either of the liquids isdesired.

Naturally, partly coated anodes may also be made in different mannersfrom that described in this example.

EXAMPLE XIV

10 cc butyl alcohol

2 cc butyl titanate

1 cc pentaethyl tantalate

1 cc pentaethyl niobate

1 g ruthenium chloride, bromide, or iodide

0.1 g hydrogen chloride

A zirconium base was degreased and pickled in a known manner. The abovemixture was painted onto the base and converted by heating at 400°-700°Cin air. This treatment was repeated until 40 g/m² of the desired mixturewas present on the surface. The mixed material consisted of the oxidesof titanium, tantalum, and niobium as protective oxides of film-formingmetals and ruthenium oxide as the electrocatalytic non-film-formingconductor.

In addition, some zirconium oxide had formed thermally on the boundarysurface of the mixture and the zirconium rod. The quantity of oxides offilm-forming metals was more than 50 mol %, calculated on the overallmixture.

Such an anode is particularly suitable for all kinds of electrolyses,such as of sulphuric acid compounds, for the purification of water, andfor the preparation of chlorates.

EXAMPLE XV

A titanium plate was degreased, pickled, and subjected to ultra-sonorousvibrations. Subsequently the plate was placed as an electrode in astirred emulsion consisting of:

100 cc water

100 cc acetone

5 g extremely finely-divided mixture of co-precipitated platinum oxide(3 g) and titanium oxide (2 g)

1 g emulsifying agent

The second electrode was constituted by a platinum plate. By applying anelectric voltage of 10 - 100 volts, the titanium was electrophoreticallycoated with a mixed oxide from the emulsion. After being removed fromthe bath, the titanium with the coating deposited thereon was carefullydried and subsequently heated at 400°C for several minutes. Thereafterthe electrophoretically deposited layer had an excellent adhesion to thetitanium, and the anode thus made is suitable for various kinds ofelectrolyses.

The adhesion is highly promoted by pre-oxidizing the titanium base bymeans of heat or electrolytically, and then applying the mixed oxide byelectophoresis.

This example was repeated using a mixture of co-precipitated platinumoxide, titanium oxide, and manganese dioxide. There is thus obtained ananode having a high overvoltage and catalytic properties.

EXAMPLE XVI

Two titanium rods were degreased and pickled and subsequently placed ina galvanic bath having the following composition:

100 cc ethanol

100 cc water

1 g ruthenium chloride

10 g titanium chloride

and subsequently connected to a source of alternating current of 13volts and a current density of 15 Amp/m², temperature 20°-30°C, for aperiod of time of about 20 minutes.

After about 20 minutes both rods were coated with a mixture of titaniumoxide and ruthenium oxide, the adhesion of which was still furtherimproved by heating at 400°C for 5 minutes.

The anode thus made is excellently suitable for use in variouselectrolyses effected at low current densities.

EXAMPLE XVII

A titanium rod was degreased and subsequently electrolytically providedwith an oxide coating of a thickness of about 5 microns. The rod thustreated was placed as an anode in a bath (80°C), containing:

100 cc water

5 g yellow lead oxide

5 g sodium hydroxide

3 cc hydrogen peroxide

10 cc titanium chloride solution (25% Ti0₂)

This bath is regularly insufflated with air. The treated titanium rodwas connected an anode, an iron plate being used as the cathode. Thevoltage differential between the anode and the cathode was about 2 - 3volts, and the current density was about 5 Amp/m².

After about an half hour, the titanium anode was coated with a mixtureof titanium oxide and lead oxide, the properties of which could beconsiderably improved by heating at 200°- 600°C.

An anode thus treated is suitable for use in electrolyses in which nohigh-current densities are necessary.

EXAMPLE XVIII

Titanium expanded metal was degreased and pickled, and then painted withthe following mixture:

10 cc butyl alcohol

1 g ruthenium chloride

3 cc zirconium acetyl acetonate.

Subsequently the product was heated at 400° - 700°C. this treatment wasrepeated until the mixture on the titanium had a thickness of 1/2micron.

An electrode thus made is excellently suitable for the electrolysis ofsolutions of sulphuric acid compounds, the resistance of the titanium tosulphuric acid being greatly increased by virtue of the mixed surfacecoating containing zirconium oxide.

EXAMPLE XIX

A tantalum wire was degreased and pickled, and then dipped into amixture of

10 cc butyl alcohol

3 cc butyl titanate

1 g iridium chloride

The wire was then heated at a temperature of 500°-700°C, and thetreatment was repeated until at least 2.5 g of the mixture of the oxidesper m² was present on the surface of the tantalum.

A tantalum wire thus treated is excellently suitable for use as an anodefor the cathodic protection of ships.

EXAMPLE XX

A titanium plate was degreased, pickled, subjected to ultrasonoricvibrations and then rinsed thoroughly and dried. This plate wassubsequently placed in an apparatus in which metals can be vacuumdeposited. As cathodes, a bar of platinum and bars of titanium wereconnected, the ambient atmosphere containing so much oxygen that thecathodic materials were deposited on the titanium as oxides (a detaileddescription of this apparatus is contained in the book by L. Holland,VACUUM DEPOSITION, 1963, pages 454-458).

After several minutes a mixture of titanium oxide and platinum oxide wasdeposited on the titanium, and the titanium thus treated is excellentlysuitable for the electrolysis of aqueous electrolytes.

EXAMPLE XXI

Niobium was degreased and subsequently provided with an oxide coating ofa thickness of at least 1 micron. This can be effected eitherelectrolytically or thermally.

Subsequently a past was prepared of:

10 cc ethanol

1 g ruthenium oxide

4 g titanium oxide.

This mixture was intimately admixed, heated, sintered, comminuted, andagain mixed with 10 cc ethanol. The resulting paste was applied to theoxidized niobium in a thin layer, and subsequently heated at atemperature of 450°- 700°C. This treatment was repeated until at least10 g of the desired mixture per m² was present on the surface.

A niobium plate thus treated is excellently suitable for theelectrolysis of electrolytes.

EXAMPLE XXII

A soft-quality titanium rod was degreased and then a mixture of morethan 50 mol % titanium oxide and less than 50 mol % palladium oxide wasrolled into it under pressure. Alternatively, this may be effected byhammering.

The oxides were prepared by dissolving water-soluble salts of the metalsin water in the required proportions, from which solution they wereprecipitated with lye, washed, and carefully dried. In this manner avery fine mixed oxide was obtained, which could be hammered or rolledinto the titanium without undue trouble. Other conventional methods ofpreparing these mixed oxides can naturally be used as well.

Furthermore, other metals than titanium can naturally be treated in thismanner.

EXAMPLE XXIII

6.2 cc butyl alcohol

0.4 cc water

3 cc butyl titanate

1 g ruthenium chloride.

The above solution was painted onto a titanium base and heated asdescribed in Example I.

An anode thus made is particularly suitable for the electrolysis of zincsulphate or copper sulphate solutions which may be contaminated withnitrate or chloride, for the manufacture of the metals concerned.

EXAMPLE XXIV

A zirconium plate was degreased and subsequently provided with thedesired mixture of oxides by means of a so-called plasma burner.

There is thus obtained a very thin, but excellently adhering layer, anda zirconium plate provided with such a coating is excellently suitablefor all kinds of electrolyses.

In the above illustrative examples, the materials from the groupconsisting of gold, silver, platinum, palladium, iridium, ruthenium,osmium, rhodium, iron, nickel, chromium, copper, lead, manganese andoxides, nitrides, carbides and sulfides thereof and graphite, act aselecrocatalytic conductors to conduct current from the anode base to theelectrolyte and in the production of chlorine to catalyze the formationof chlorine molecules from the chloride ions released at the anode, andthe oxides and other compounds of the film forming metals act to protectthe electrocatalytic material and promote adherence of the coatings tothe base.

The nitrides, carbides, sulfides and graphite given by way ofillustration in Example XII act as electrocatalytic agents protected bythe titanium oxide. The combination of electrocatalytic agents andprotective oxides has excellent adherence to the anode base.

Various modifications and changes may be made in the steps described andthe solutions and compositions used without departing from the spirit ofthe invention or the scope of the following claims.

What is claimed is:
 1. An electrode for use in an electrolytic reactioncomprising a base of electrically conductive film-forming metal in solidform having thereon a mixed coating material of an electricallyconductive coating material effective for carrying out electrolysis andprotecting coating material consisting of at least one metal oxide of asingle film-forming metal, the protecting coating material acting duringelectrolysis in a cell to protect the conductive coating materialagainst the cell conditions, the conductive coating material being atleast one material from the group consisting of gold, silver, platinum,palladium, iridium, ruthenium, osmium, rhodium, iron, nickel, chromium,copper, lead, manganese, and nitrides, carbides and sulfides thereof,and said group further consisting of oxides of gold, silver, iron,nickel, chromium, copper, lead and manganese.
 2. An anode as claimed inclaim 1 in which the base is metal and the protecting coating materialis an oxide of the metal constituting the base.
 3. An anode as claimedin claim 1 in which the protecting coating material is from the groupconsisting of oxides of aluminum, tantalum, titanium, zirconium,bismuth, tungsten and niobium.